Epact In determining the epact we either find the number of days required to make up the lunar to the solar year, and so the numeral of the moon's age on January 1, or, with Scaliger, we may use March 1, which comes to the same thing, and has the advantage of avoiding the ambiguity of leap-year.
The old Latin cycles of eighty-four years indicated Easter by means of the epacts of January 1, and the day of the week on which January 1 fell.
The method of determining the months (lunar) was as follows: For the first month of the year, that month was taken whose age was expressed by the epact. The day of December on which it commenced is found by subtracting the epact (when more than one) from thirty-three. The first month was always counted full, then hollow and full succeeded by turns, so that the last month in the year, in a common lunar year, was hollow, in an intercalary year full. From the last begins the new moon of the following year.
The Easter new moon being found, Easter-day was, according to the Latin rules, that Sunday which fell on or next after the 16th of the moon, not therefore later than the 22d of the moon. The choice of the month was determined thus: New moon must not be earlier than March 5, and full moon not later than March 21; the first of these rules sometimes having to give way to save the violation of the latter.
The following rule is given for the epact of January 1, viz., multiply the golden number by eleven, and divide the product by thirty, the remainder is the epact. But this rule will not give the epacts mentioned above, which were constructed as we have just described with a saltus lunae, or addition of twelve after the 19th year of the cycle, etc.